I've already out in my two cents earlier but just looking over several of the pages I can already see things have gotten into an in-depth discussion.
Ultimately, though, I think Mr. Raven's idea (whether it was his intent or not) is a good one: Separate systems for single and multi-player.
Furthermore, though, I think that if the tactical battles are /fun/ I wouldn't mind them going on forever, and if they all end artificially and in a contrived fashion I'll be a little miffed.
Options would be a good idea, leave the option open for people who like longer battles and more advanced tactics, but for people with less time and patience allow the option for forced retreat after N turns, sliders, check-boxes, etc.
Ultimately; I like being able to charge in and attack them retreat, taking no losses but slowly whittling the enemy down, and Winner Take All, as has been already emphatically stated by several, pretty much cuts your options, makes battles all about decisive clashes, and enhances the steamroller effect that makes most Strategy Games boring and un-challenging.
Still, though, some ideas; Perhaps units based on their speed and mobility have a 'retreat cost' and morale also plays into this somewhat, morale based on losses.
In this case, you could build units, like say, cavalry and skirmishers, who can retreat more effectively, and with good morale, won't 'lose' when they retreat, and these units could be used for harassment, while other less mobile units with lower morale would 'break' when they retreated, or be open to attack.
Maybe that would help keep options open, but make it so the players who don't have the time don't have to engage in long campaigns? Either way, I'd really like more in-depth combat, but I trust you guys to make something fun in the end either way.